HOW TO USE A TRANSISTOR AS AN AMPLIFIER | transistor characteristics and parameters

HOW TRANSISTOR WORKS AS AN AMPLIFIER

As we know transistor is a current control device, it is used for controlling the current in electronics circuits.Transistor is an electronic device used in a circuit to control a large amount of current or voltage with a small amount of voltage or current.

Two layers sandwiching one semiconductor between two other semiconductors. Because the current is transferred across a material that normally has high resistance (i.e. a resistor), it was a "transfer-resistor" or transistor.
The first practical  transistor was made  in 1948  by Shockley,

How a transistor works

for understanding how a transistor works, you have to understand how semiconductors react to an electric potential. Some semiconductors will be n-type, or negative, which means that free electrons in the material drift from a negative electrode (of, say, a battery it's connected to) toward the positive.

                                                                                     Now, we are taking the example of  a npn transistor. Each end of the transistor is an n-type semiconductor material and between them is an p-type semiconductor material. If you picture such a device plugged into a battery, you'll see how the transistor works:

• We are taking npn transistor because of in this the mobility of charge carriers is high as compare to PNP transistor.
• the n-type region attached to the negative end of the battery helps propel electrons into the middle p-type region.
• the n-type region attached to the positive end of the battery helps slow electrons coming out of the p-type region.
• the p-type region in the center does both.

PNP BJT Common Emitter Characteristic

Bipolar Junction Transistors

Bipolar Junction Transistors

Common Emitter Amplier

DC analysis: An emitter resistor is necessary to provide stability of the bias point.  We need to include RE for good biasing (DCsignals) and eliminate it for AC signals.The solution to include an emitter resistance and use a "bypass" capacitor to short it out for AC signals as is shown.

AC analysis of bjt

To start the analysis, we kill all DC sources, combine R1 and R2 into RB and replace the BJT with its small signal model. We see that emitter is now common between input and output AC signals (thus, common emitter ampli er. Analysis of this circuit is straightforward. Examination of the circuit shows that:

BJT current and voltage Equations :-

npn                                                                      pnp

IE = IB + IC                                                IE = IB + IC

VCE = -VBC + VBE                                         VEC = VEB - VCB

Modes of Operation of bjt

Active region of bjt:      

·         Most important mode of operation

                  ·         Central to amplifier operation

                    ·         The region where current curves are practically flat

         Saturation region of bjt:

•   Barrier potential of the junctions cancel each other out causing a virtualshort
• 
  

         Cutoff region of bjt :

•Current reduced to zero

•Ideal transistor behaves like an open switch